1. Field of the Invention
This invention relates to circuit breakers, and in particular to draw-out circuit breakers mounted in an enclosure provided with a shutter to block access to the distribution system conductors when the circuit breaker is removed from the enclosure. The invention further relates to an interlock for interlocking two such draw-out circuit breakers to prevent a condition in which both breakers are closed at the same time.
2. Background Information
Low voltage circuit breakers used in electric power distribution systems are commonly mounted in metal enclosures and supported on rollers which permits easy installation and removal of the sizeable circuit breakers required for such service. Typically, the line and load conductors are terminated in the rear wall of the enclosure with stabs which are engaged by finger connectors on the back of the circuit breaker as it is racked into the enclosure. In order to protect personnel from the exposed power system terminations when the circuit breaker is removed, shutters have been developed which automatically close over the stabs as the circuit breaker is withdrawn, and which are opened by inward movement of the circuit breaker during installation.
In some applications, such as, for example, transfer switches which provide alternate power sources to a load, seeps must be taken to ensure that alternate sources with random phase relationship are not connected to the load at the same time. Interlocks have been developed which prevent more than one switch from being closed at any given time. Often, circuit breakers are used as the switches which must be interlocked, and this includes the draw-out low voltage circuit breakers. One form of interlock for such circuit breakers is the cable interlock. Typically, this interlock includes a pivoted arm mounted behind the circuit breaker which pulls on a cable when rotated by a plunger which extends from the back of the circuit breaker when the contacts of that circuit breaker are closed. The cable actuates a device on the other circuit breaker which prevents the contacts of that other circuit breaker from being closed. A similar cable interlock prevents the first circuit breaker from being closed when the other circuit breaker is closed.
It is common for the cable interlock to hold the second circuit breaker in a tripped condition to prevent closure of the contacts. When the first circuit breaker is removed from its enclosure, the interlock mechanism is exposed. Typically, in the case of a transfer switch under these conditions, the second circuit breaker would be closed to connect the alternate power source to the load. Usually, it is important in such applications that power be maintained to the load. However, with the interlock exposed by removal of the first circuit breaker, it is possible that a worker could inadvertently or otherwise actuate the interlock device and trip the second circuit breaker, thereby, interrupting power flow to the load.
Currently, there is no draw-out low voltage circuit breaker having both a shutter and an interlock, as these two mechanisms compete for space behind the circuit breaker. However, there is a need for a draw-out low voltage circuit breaker which provides protection for personnel when the circuit breaker is removed and which when used with another circuit breaker provides that both circuit breakers cannot be closed at the same time. There is a further need for such a circuit breaker unit in which the interlock cannot be actuated when the circuit breaker is removed from the enclosure to prevent an inadvertent or intentional tripping of the other circuit breaker to which the interlock is connected.